This invention relates to magnetic recorders and recording methods and especially to such recorders adapted to be used to record and reproduce individual video frames for stop-action or slow motion video display. The standard for video signals currently used in the United States requires that a scanning pattern of 525 horizontal scanning lines per frame be utilized. A complete frame consists of two interlaced fields, the first field including the even numbered lines and the second field including the odd numbered lines in the picture scan. All of the lines in the first field are scanned in succession before the lines in the second field are scanned.
Magnetic disc recorders have in the past been used to produce high resolution images of single video frames. An individual frame may be reproduced repeatedly and supplied to a monitor so that a stop-action effect is created. Successive video frames each may be supplied to a video monitor a number of times to create a slow motion effect. If the subject portrayed in the video frame is in rapid motion, however, the first and second fields may differ significantly and interlaced display of these fields may therefore cause significant distortion. An effect known as motion jitter may occur due to the spatial displacement of the subject during the elapsed time between the two interlaced fields.
One approach to this problem has been to record only one field and to display this field repeatedly at the field rate. A frame thus would consist of only one field which is displayed twice in interlaced fashion. As shown in U.S. Pat. No. 3,636,253 to Notani et al., issued Jan. 18, 1972, this may be accomplished by using a field recorder in which one field is recorded during one full rotation of the magnetic disc. A frame is developed by reproducing the field signal twice and by displacing the second reproduced field by one-half horizontal line scan time from the first reproduced field to create the proper conditions for interlace. Since the first and second fields are identical, however, vertical resolution is needlessly sacrificed in displaying still subjects or subjects which are moving relatively slowly.
To allow for high vertical resolution in the case of still subjects and jitter-free operation in the case of rapidly moving subjects, two methods have previously been utilized. The first method is to require the disc recorder to operate at two rotational speeds. If a rapidly moving subject is to be displayed, the recorder is operated at the field rate with one field repeatedly displayed in interlaced fashion. If a still or slowly moving subject is displayed, however, the disc recorder is operated at the frame rate with both fields of a frame being recorded and displayed. This method of operation is difficult to achieve and relatively expensive. The disc drive servo-mechanism for two speed operation is complicated and sophisticated controls are required. Further, differences in dynamic performance of the recorder at the two speeds must be considered, both in the case of flying head hard disc recorders and in the case of flexible disc recorders.
A second method shown in U.S. Pat. No. 3,518,366 to Phan, issued June 30, 1972, involves rerecording one field of a frame twice on a frame recorder. To accomplish this, a frame recorder is modified by providing a second recording head precisely displaced from the first head. The two magnetic heads then record the single field simultaneously and one head is used for playback of the frame. If the disc is a multi-track disc, an additional stepper mechanism is required for the extra head. This method may also be costly and complicated and may cause excessive disc wear.